Method for particle beam testing of substrates for liquid crystal displays (LCD)

ABSTRACT

Method for particle beam testing of substrates for liquid crystal displays (LCD). This is directed to methods wherein, given a substrate (SUB1) for a liquid crystal display, either potentials or, respectively, currents are set in defined fashion with a particle beam (S1, S2 and S4) and/or potentials are measured by detecting secondary electrons (S5) at different switch statuses of the switch elements (T) of the substrate (SUB1). The geometrical integrity and the electrical functionability of the substrate (SUB1) are thereby tested, even though, for example, a supplementary plane electrode is not present for forming a capacitor. An important advantage of the method is that faulty substrates can be repaired or can be segregated even before further-processing and, thus, costs can be reduced.

This is a division of application Ser. No. 882,657, filed May 13, 1992,now U.S. Pat. No. 5,268,638.

BACKGROUND OF THE INVENTION

The present invention is directed to a method for testing a substratefor a liquid crystal display that has a plurality of picture elements,whereby the substrate is composed of a light-transmissive insulatormember and whereby a plurality of plane electrodes, switch elements andcontrol lines are applied onto the surface thereof, such that arespective plane electrode is connected to control lines via a switchelement.

A method for non-contacting testing of line networks for shorts andinterruptions is disclosed by European Patent reference EP 0 189 777 B1(corresponding to U.S. Pat. No. 4,985,681).

SUMMARY OF THE INVENTION

The object of the present invention is to provide an improved methodthat allows a testing of the geometrical integrity, as well as, atesting of the electrical function, even though, for example, asupplementary plane electrode for forming capacitors is not situated onthe substrate to be tested.

This object is inventively achieved in that respective control lines arebrought to a defined potential and in that the resulting potential ofthe appertaining plane electrode is measured with secondary electrondetection.

According to a further aspect of the present invention, a respectiveplane electrode has a defined current supplied to it by a particle beamand the resulting potential of this plane electrode is measured bysecondary electron detection.

According to another aspect of the present invention, alternatively, arespective plane electrode has a defined current supplied to it and thecurrents resulting therefrom are measured in the appertaining controllines.

The advantage of the present invention is particularly that faultysubstrates can be repaired or can already be eliminated beforefurther-processing and, thus, costs can be eliminated.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the present invention which are believed to be novel,are set forth with particularity in the appended claims. The invention,together with further objects and advantages, may best be understood byreference to the following description taken in conjunction with theaccompanying drawings, in the several Figures in which like referencenumerals identify like elements, and in which:

FIG. 1 is a perspective sectional view of a liquid crystal display thatis composed of two substrates and of an intervening liquid crystal;

FIG. 2 depicts a substrate under test on which parts of picture elementsare applied; and

FIG. 3 is an electrical equivalent circuit diagram of a picture element.

DESCRIPTION OF THE PREFERRED EMBODIMENT

As may be seen from FIG. 1, liquid crystal displays (LCDs) areessentially composed of two substrates SUB1 and SUB2 between which aliquid crystal LC is enclosed. In the illustrated case, a plurality ofplane electrodes are applied on the substrate SUB1 and a supplementaryplane electrode CE is applied on the substrate SUB2. Together with, forexample, the supplementary opposed plane electrode CE of the substrate,the plane electrode PE forms a capacitor in whose field long-chainliquid crystal molecules are aligned, as a result whereof the liquidcrystal becomes light-transmissive in the region of the plane electrodePE. When separately testing the substrate SUB1 from the substrate SUB2,the intended mode of operation encounters difficulties since thecapacitances formed by the plane electrodes, the liquid crystal and thesupplementary plane electrode CE are not present.

FIG. 2 shows only the substrate SUB1 with a plurality of planeelectrodes switch elements in the form of transistors control lines,whereby gate terminals of transistors of a are connected matrix-like toa control line, source terminals of the MOS transistors of a column areconnected matrix-like to a control line and the respective planeelectrodes of the picture elements are connected matrix-like to thedrain terminals of the MOS transistors. For example, the plane electrodePE of a picture element is connected to the drain terminal of atransistor T whose gate terminal is contacted by a control line L2 andwhose source terminal is contacted by a control line L1. The line L1 hasa line terminal A1 and the line L2 has a line terminal A2. A current inthe line L1 is referenced I1 and a current in the line L2 is referencedI2. A particle beam S1 is directed onto the line L1 and a particle beamS4 is directed onto the line L2. A particle beam S2 is directed onto theplane electrode PE itself in order to supply a current I to the planeelectrode PE. A particle beam S3 that causes secondary electrons S5 isdirected onto the plane electrode PE separately from the particle beamS2. The separate particle beams S2 and S3 thereby indicate that, forsetting a potential or a defined current I, these need not be identicalwith the particle beam S3 in terms of their properties, the latterserving the purpose of triggering secondary electrons. Thus, forexample, it is conceivable that the particle beam S2 delivers a particlebeam S3 that serves the purpose of measurement or that the particle beamS2 is composed of an electron beam and the particle beam S3 is composedof a laser beam that generates secondary electrons S5 in the form ofphoto electrons.

FIG. 3 shows an electrical equivalent circuit diagram of a pictureelement with a MOS transistor T as a switch element whose sourceterminal is contacted by the control line L1 and whose gate terminal iscontacted by the control line L2. A drain terminal of the MOS transistorT is connected to the plane electrode PE that, together with the commonplane electrode CE, forms a capacitor C of the picture element, wherebythe supplementary plane electrode CE is connected to the supply voltageterminal V. A section line that separates the two plane electrodes PEand CE from one another indicates the circuit division onto twosubstrates. FIG. 3 further depicts, the essential, parasitic switchelements, in the form of parallel connected resistor R1 and capacitorC1, between the drain terminal of the MOS transistor T and the controlline L1, as well as, a parallel circuit composed of a resistor R2 and acapacitor C2 between the drain terminal of the MOS transistor T and thecontrol line L2.

In addition to the described transistors, diodes or what are referred toas MIM elements (metal-insulator-metal) are also often used as switchelements, whereby, for example, a diode is respectively provided betweena control line and the plane electrode of an image element or a MIMelement is inserted between the plane electrode and a control line. Thedivision of the switch elements and of the plane electrodes onto the twosubstrates SUB1 and SUB2 can also be implemented differently and thepresent

In a first method of the present invention, control lines L1 and L2 thatare connected via the MOS transistor T to the plane electrode PE arebrought to a defined potential at every picture element within a settingtime interval, whereby this can occur, for example, via the lineterminal A1 or, respectively, A2 or via the particle beam S1 or,respectively, S4. As a result of the parasitic elements R1, C1, R2 andC2 shown in FIG. 3 and as a result of the MOS transistor T, acorresponding potential is established at the plane electrode PE. Formeasuring the potential of the plane electrode PE in this method, aparticle beam S3 is directed onto the plane electrode PE within ameasuring time interval and the secondary electrons S5 thereby triggeredare identified. The plurality of secondary electrons that are incidentinto a detector is thereby dependent on the potential of the planeelectrode PE since the forces of attraction or repulsion depend on thepotential of the plane electrode PE. Deviations from the respectiverated potential result for faulty picture elements and these, forexample, can be easily interpreted with an electronic calculator. Whenlines are charged by a particle beam, then the charging must becyclically repeated since the lines gradually discharge. The settingtime interval is thereby selected such that the potentials on thecontrol lines change in potential only within a defined measuringtolerance. The measurement time interval must thereby be optimally shortso that no significant change in potential arises on the plane electrodePE.

A second inventive method for particle beam testing of a substrate for aliquid crystal display is that the defined current I is supplied to theplane electrode PE by the particle beam S2 and the secondary electronsS5 are triggered the particle beam S3 that is likewise directed onto theplane electrode PE and, thus, the potential of the plane electrode ismeasured. In many instances, the particle beam S2 will be identical tothe particle beam S3 since the particle beam for supplying the current Iis also suitable for triggering secondary electrons. The current I iscarried off by the parasitic elements R1, C1, R2 and C2 indicated inFIG. 3 and by the switch element T, so that a defined potential isestablished on the plane electrode PE. This established potential can berespectively compared to a rated potential and a test result for apicture element can be formed therefrom.

In a third inventive method for particle beam testing of a substrate fora liquid crystal display, a defined current I is likewise supplied tothe plane electrode PE. Here, however, it is not the potential of theplane electrode but the currents I1 and I2 that arise due to theparasitic elements R1, C1, R2 and C2 and the transistor T that aremeasured. The measurement of the currents I1 and I2 occurs via the lineterminals A1 and A2 that are connected to external measuring contacts.In this method, for example, the measuring time interval can also liewithin the setting time interval, i.e. the current I can also besupplied during the measuring time interval in order to obtain astationary division of current.

In all three methods of the present invention, a measurement can occureither immediately after a setting time interval or after a definedwaiting time. When the measurement is not carried out until after adefined waiting time, then it is errors in the time behavior of apicture element that are particularly identified. When the measurementis carried out after the setting time switch element during themeasuring time interval via the connected control lines. The controllines can thereby be set via the terminals A1 or, respectively, A2 orvia the particle beam S1 or, respectively, S4.

The three methods of the present invention can also be combined, wherebya defined potential is produced on the line L2, for example via terminalA2, a defined current I is supplied to the plane electrode PE via aparticle beam S2, the current I1 of the line L1 is measured via aterminal A1 and the potential of the plane electrode PE is detected viathe secondary electrons S5 triggered by the particle beam S2.

An electron beam is especially suited as the particle beam. However, thephoton beam of a laser can be used that produces secondary electrons inthe form of photo electrons. An ion beam is also often used forrepairing extremely fine interconnect structures and this also triggerssecondary electrons. A testing method of the present invention cantherefore be combined with a repair by utilizing an ion beam for theafore-mentioned jobs.

The invention is not limited to the particular details of the methoddepicted and other modifications and applications are contemplated.Certain other changes may be made in the above described method withoutdeparting from the true spirit and scope of the invention hereininvolved. It is intended, therefore, that the subject matter in theabove depiction shall be interpreted as illustrative and not in alimiting sense.

What is claimed is:
 1. A method for particle beam testing of substratesfor liquid crystal displays comprising the steps of: providing asubstrate for a liquid crystal display that has a plurality of pictureelements, each of said picture elements having a switch element, thesubstrate having on a surface thereof a plurality of plane electrodes,switch elements of picture elements and control lines such thatrespectively one plane electrode of the plurality of plane electrodes isconnected via at least one switch element to control lines; supplying afirst current to a predetermined plane electrode by means of a firstparticle beam within a respective setting time interval, the firstcurrent being conducted by parasitic elements and a respective switchelement associated with the predetermined plane electrode so that apotential is established by the predetermined plane electrode; detectingsecondary electrons triggered by one of the first particle beam or asecond particle beam and thereby measuring the potential of thepredetermined plane electrode within a respective measuring timeinterval; and comparing the measured potential to a respective ratedpotential and forming therefrom a test result of a picture element suchthat a picture element is identified as faulty when a value of themeasured potential substantially deviates from a value of the respectiverated potential.
 2. The method according to claim 1, wherein themeasuring time interval immediately follows the setting time interval.3. The method according to claim 2, wherein the switch element isswitched within the measuring time interval via the control linesconnected to said switch element.
 4. The method according to claim 1,wherein a defined waiting time is provided between the end of thesetting time interval and the beginning of the measuring time interval.5. The method according to claim 4, wherein the switch element isswitched within the measuring time interval via the control linesconnected to said switch element.